A spark plug is mounted on an internal combustion engine (sometimes just referred to as “engine”) etc. and used to ignite an air-fuel mixture in a combustion chamber of the engine. In general, the spark plug includes an insulator having an axial hole formed in an axis direction, a center electrode inserted in a front side of the axial hole, a metal shell arranged on an outer circumference of the insulator and a ground electrode joined to a front end portion of the metal shell. The ground electrode is bent at a substantially middle position thereof such that a distal end portion of the ground electrode faces a front end portion of the center electrode so as to form a gap between the distal end portion of the ground electrode and the front end portion of the center electrode. With the application of a high voltage to the gap, the spark plug generates a spark discharge for ignition of the air-fuel mixture.
For improvement in the oxidation resistance of the ground electrode, there has recently been proposed a technique to cover a center-electrode-side gap-forming part (referred to as “discharge part”) of the ground electrode with a protection layer of highly oxidation-resistant metal (see e.g. PCT International Application Publication No. JP-T-2009-533802 and PCT International Application Publication No. JP-T-H11-514145). In particular, it has been proposed in PCT International Application Publication No. JP-T-2009-533802 to cover the entire surface of the ground electrode with the protection layer.
During operation of the internal combustion engine, a part of the ground electrode located closer to a center side of the combustion chamber than the discharge part and protruding more from a front end of the metal shell reaches a particularly high temperature. Among the distal end portion of the ground electrode, a distal end face and an outer circumferential surface other than the center-electrode-side surface of the ground electrode become particularly high in temperature and tends to get corroded by oxidation. Thus, the oxidation resistance of the ground electrode may not be improved sufficiently even in the case where the protection layer is formed on the discharge part of the ground electrode.
In the case where the entire surface of the ground electrode is covered with the protection layer, by contrast, the ground electrode can achieve high oxidation resistance. However, the protection layer causes deterioration in thermal conductivity because the constitutional material of the protection layer contains additives such as chromium and aluminum for improvement in oxidation resistance. It is thus difficult to radiate heat of the ground electrode so that the heat radiation performance of the ground electrode becomes deteriorated in the case where the entire surface of the ground electrode is covered with the protection layer. As a result, there is a possibility of overheating of the ground electrode, which leads to pre-ignition by the action of heat from the ground electrode as well as wear resistance deterioration of the ground electrode.
The present invention has been made in view of the above circumstances. An advantage of the present invention is a spark plug in which a ground electrode can be assuredly prevented from overheating while securing sufficient improvement in oxidation resistance.